I want to use my phone as an alarm clock but use vibrate only. Do I need to put it in airplane mode? If it's in airplane mode, do I turn the phone off before going to bed or let it sleep? Or do I put it in silent mode? Thanks!

The only way I think this is possible is if you have the option to individually change the volume of the alarm ringer and enable the vibrate function. I use a Nexus 5X and I do have that option but it may not be available for all phones.